Decorator Pattern

What

Dynamically adds new behavior or responsibilities to objects without altering their structure. Wraps an object to provide new behavior.

When

You need to add behavior to objects without affecting others of the same class. You want to add behavior dynamically without subclassing.

class PlainText {
    public String render() {
        return "Hello";
    }
}
 
class BoldText extends PlainText {
    public String render() {
        return "<b>" + super.render() + "</b>";
    }
}
 
class ItalicBoldText extends BoldText {
    public String render() {
        return "<i>" + super.render() + "</i>"; // Combos require new subclasses
    }
}
 

How

• Create a base component interface.
• Implement core component and decorator classes.
• Decorators implement the interface and wrap components.

interface Text {
    String render();
}
 
class PlainText implements Text {
    public String render() { return "Hello"; }
}
 
class BoldDecorator implements Text {
    private Text text;
    public BoldDecorator(Text t) { this.text = t; }
    public String render() { return "<b>" + text.render() + "</b>"; }
}
 

Examples

• Adding scrolling, borders, or shadows to GUI widgets.
• Adding encryption or compression to a stream.

Why

Advantages

• Flexible alternative to subclassing.
• Multiple decorators can be combined.
• Promotes single responsibility and open/closed principles.

Disadvantages

• Many small classes.
• Can get complex when used extensively.